The correct answer is: C. Equator and zenith.
The declination of a star is its angular distance north or south of the celestial equator. The zenith distance of a star is the angle between the star and the zenith, which is the point directly overhead. The latitude of a place is the angle between the Earth’s equatorial plane and the vertical at that place.
If the latitude of a place is obtained by subtracting the declination of a star from its zenith distance, then the observed star must be between the equator and the zenith. This is because the zenith distance of a star at the equator is 0 degrees, and the zenith distance of a star at the pole is 90 degrees. Therefore, the zenith distance of a star at any latitude between the equator and the pole must be between 0 degrees and 90 degrees.
Option A is incorrect because the horizon is the line that separates the sky from the ground. The zenith distance of a star at the horizon is 90 degrees, so the observed star cannot be between the horizon and the equator.
Option B is incorrect because the zenith is the point directly overhead. The zenith distance of a star at the zenith is 0 degrees, so the observed star cannot be between the zenith and the pole.
Option D is incorrect because the pole is the point on the Earth’s surface that is directly above the North Pole or the South Pole. The zenith distance of a star at the pole is 90 degrees, so the observed star cannot be between the pole and the horizon.